Calculation of recurrence risks for heterogeneous genetic disorders

We present general formulae for several common situations in the genetic counseling of heterogeneous disorders. The occurrence or not of parental consanguinity is taken into account, since it distorts significantly the prior probabilities favoring the different mechanisms. Nonsyndromic deafness is used as a numerical application, since it can be produced by any type of monogenic inheritance and can be mixed with variable proportions of environmental cases. Recurrence risks are calculated including or not including environmental factors in the origin of the defect. In underdeveloped countries the proportion of environmentally determined cases of deafness is significantly higher than in first-world countries. Therefore, when environmental causes cannot be excluded, recurrence risks are always higher in developed than in developing countries. On average, when parental consanguinity is present there is a significant increase in recurrence risks for deafness, whether environmental factors are included or not.

Why is the reproductive performance lower in Becker (BMD) as compared to limb girdle (LGMD) muscular dystrophy male patients?

We had previously reported that patients affected with BMD have a significantly reduced reproductive performance (f = 0.12) as compared to male LGMD patients of similar age and physical impairment (f = 0.98). In the present study parameters such as the socio-economic level, as well as psychosocial, intellectual, and psychiatric functionings could not explain the low fitness of BMD patients. The effect of genetic counseling, a greater difficulty in coping with the disease, and relating to women and/or a potential malfunction of reproductive physiology are discussed as possible causes.

Becker and limb-girdle muscular dystrophies: a psychiatric and intellectual level comparative study

There are some indications that Becker muscular dystrophy (BMD) might be related to mental disorders and mental retardation (MR). To investigate this question, we made a standardized psychiatric and intellectual level assessment of 22 BMD patients in comparison with 22 limb-girdle muscular dystrophy (LGMD) patients. There were not significant differences between the two groups. Twelve patients (54.5%) in each group received at least one lifetime psychiatric diagnosis, the most frequent being depressive disorders. The intelligence quotient means for BMD was 85.9 and 87.8 for LGMD. There was one case of mild MR among BMD patients and two cases among LGMD patients.

Phenotypic spectrum of 45,X/46,XY individuals

We report on five patients with 45,X/46,XY mosaicism. In these subjects, as well as in 58 individuals from the literature with a similar chromosome constitution, we did not find a preponderance of 46,XY cells among patients showing ambiguous to abnormal male external genitalia when compared to those patients with slight or no virilization. However, the average frequency of 46,XY cells in blood in these mosaic individuals suggests that this sample includes mainly individuals whose mosaicism originated early in embryonic cell division. Those individuals whose mosaicism originated later are not significantly represented in this sample and would have higher frequencies of 46,XY cells. These individuals would be excluded from an intersex sample if they had well-virilized genitalia. This ascertainment bias suggests that the degree of virilization depends on the frequency of 46,XY cells.

Treatment of Duchenne muscular dystrophy with growth hormone inhibitors

A controlled, double-blind therapeutic trial with the drug mazindol, a growth hormone inhibitor, was performed in a pair of 7 1/2 year-old monozygotic twins, with Duchenne muscular dystrophy (DMD). The rationale for this trial was based on a patient (reported previously) affected simultaneously with DMD and growth hormone (GH) deficiency, who is showing a benign course of the dystrophic process and is still walking at 18 years. One of the twins received 2 mg of mazindol daily, while the other received a placebo. The assessment, repeated every 2 months, included weight and height measurements, functional and motor ability tests, ergometry and determinations of serum enzymes and GH levels. After one year of trial the code was broken and it was seen that the twin under placebo treatment was strikingly worse than his brother, the progression of whose condition was practically arrested. These results strongly suggest that treatment with a GH inhibitor is beneficial for DMD patients.

A Bayesian method for the estimation of penetrance: application to mandibulofacial and frontonasal dysostoses

We describe a Bayesian method of estimating penetrance from genealogical data. It consists of calculating the likelihood of the data alone to make inferences about penetrance without sample space considerations. The method is applied to mandibulofacial dysostosis giving a penetrance of 0.908 with 0.95 credible interval of [0.809; 0.972] and to frontonasal dysostosis giving a penetrance of 0.670 with 0.95 credible interval of [0.457; 0.851].

Testicular regression in a patient with virilized female phenotype

The 16-year old girl studied here had ambiguous external genitalia, ie, enlarged clitoris, pseudo vagina, and rudiments of Wolffian tubes. Her karyotype was 46,XY and she was H-Y antigen-positive. In spite of absence of gonadal tissue, genital virilization suggests presence of testes during embryogenesis. This patient is compared to 20 others with testicular regression from the literature. Autosomal-recessive inheritance of this condition is proposed.

Late infantile autosomal recessive myotonia, mental retardation, and skeletal abnormalities: a new autosomal recessive syndrome

Four sibs of non-consanguineous parents who had myotonia from late infancy are described. Mild to moderate mental retardation, severe bone abnormalities of the vertebral column (mainly in the thoracolumbar region), and short stature were also observed. Autosomal recessive inheritance is demonstrated. These cases are compared with reported cases of the Schwartz-Jampel syndrome.

Autosomal dominant late adult spinal muscular atrophy, type Finkel

We describe clinical and genetic data from the study of two families with 80 members affected with the autosomal dominant, slowly progressive spinal muscular atrophy of late onset (average 48.8 years), first described by Finkel in 1962. Electromyography and muscle biopsy of a number of patients confirmed the neurogenic nature of the conditions. Unusual findings in this disorder were cramps, spontaneous fits of suffocation, and symptomatic myotonia. Other manifestations are slow loss of muscle strength and progressive proximal atrophy, which starts in the lower limbs and progresses to the upper limbs; hypoactive or absent tendinous reflexes; and generalized fasciculations. Sensory and cranial nerve function is unimpaired. Probabilities for genetic counseling are evaluated by means of a method adequate to the late-onset nature of the condition.

H-Y antigen expression in a case of XX true hermaphroditism

Cells from an XX true hermaphrodite expressed a reduced amount of H-Y antigen when compared with normal XY cells and with cells from his father, who had an XY/XX chromosomal constitution. His mother had a normal karyotype and was H-Y negative. The four brothers of the patient were clinically and karyotypically normal. An X-Y interchange followed by random inactivation of the X chromosome is proposed to explain the H-Y antigen titer found in the patient.

Recurrence risks for Down syndrome

The recurrence risks for Down syndrome due to an inherited translocation are estimated from empirical data in the literature for two maternal age groups: mothers under 30 and mothers 30 and over. These risks were found to be approximately 0.3% and 0.05%, respectively. The probability for two Down syndrome sibs both having an inherited translocation was estimated as about 18.2% for the former age group and 2.7% for the latter. The relative effectiveness in preventing. Down syndrome births by karyotyping affected children is discussed.

H-Y gene expression in apparent absence of the long arm of the Y chromosome

H-Y antigen expression was detected on cells from an individual having a presumptive 45,X/46,X,i(Yp) karyotype, but was absent on cells from another person having a 46,X,i(Yq) karyotype. This suggests that the short arm of the human Y chromosome is essential for H-Y antigen expression, at least in the subjects studied.

H-Y antigen in Swyer syndrome and the genetics of XY gonadal dysgenesis

The H-Y antigen is a plasma membrane antigen involved in the organogenesis of the mammalian testis. Its expression on human cells is determined by a Y-linked gene. Phenotypic females affected by 46,XY gonadal dysgenesis (Swyer's syndrome) can be either H-Y-positive or H-Y-negative. In this paper we report H-Y antigen and endocrine studies in a sibship with three affected sisters. Immunological studies were performed on two of the patients, and a clearly positive expression was detected in both cases. Endocrine studies consisted in the investigation of the hypothalamic-pituitary-gonadal axis, which revealed that gonadal hormone insufficiency is the only endocrine abnormality associated with the syndrome. A new genetic interpretation and calssification of XY gonadal dysgenesis is proposed.

Risks of manifestation of Huntington chorea

A methodology allowing the estimate of risks heterozygosity and manifestation of Huntington's chorea (HC) within preestablished periods for individuals of any age was developed. For this, the procedure was the following: 1. The variables which could influence the distribution of age frequencies were studied. 2. The error due to the exclusion of the heterozygotes dying before the manifestation of the disease from the distribution of Wendt et al. (1959) of affected persons was corrected separately for each sex. 3. The frequency of non-affected heterozygotes in each age group was calculated separately for each sex, from the corrected distribution. From those data estimates of the following parameters could be reached: a) the probability of heterozygosity for consultants of any age; b) the risks of manifestation for the gene in a given individual within a determined period, taking into account his probability of heterozygosity and survival. 4. The risks of manifestation were tabulated for making easier their use in concrete cases. Simplified tables with approximate risks are also presented for the more common cases in genetic counseling. 5. The methodology presented allows a discrimination between the probability of heterozygostiy and the risk of manifestation. This makes possible the estimate of risks of becoming affected, taking into account not only the age of the consultant at the moment, but also the period of time starting with the consultation, to which the risk applies. This is essential for adequate genetic counseling.

The human Y chromosome: its routine identification and variability

Different criteria for identifying the human Y chromosome using only conventional staining techniques were evaluated and a procedure based on three of them was developed. It leads to correct identification in 93% of cells even without resorting to differences in size between the Y and the other G chromosomes. The variability in size of the human Y chromosome was assessed in a random group of White men. It was found to be significantly greater than that of all other chromosomes in the karyotype when the comparisons were made in such a way as to avoid the size-dependent component of variation.

Myotonic dystrophy, syringomyelia, and 2/13 translocation in the same family

The present report describes a sibship with 2 individuals affected by myotonic dystrophy and a third with syringomyelia. The mother was affected by myotonic dystrophy. A balanced 2/13 translocation was detected in the individual with syringomyelia, in one affected by myotonic dystrophy and in their clinically normal father. The association between the phenotypic anomalies and the chromosome alteration is coincidental.

The variation of recurrence risks with penetrance for isolated cases of autosomal dominant conditions

The relationship between the penetrance and the recurrence risk for sibs of isolated cases of autosomal dominant conditions with incomplete penetrance is presented.

A study of chromosomes of schistosomiasis patients under oxaminiquine (UK 4271) treatment

Blood samples from 24 patients infested with Schistosoma mansoni were drawn immediately before and 2 days after the administration of a single therapeutic dose of 12-15 mg/kg oral oxaminiquine. Two-day lymphocyte cultures were obtained and about 100 mitoses from each blood sample were analyzed for chromosome aberrations. No significant differences were observed between the "before" and "after" cultures in the frequencies of aberrations resulting from spindle, chromatidic, or chromosome events. It is concluded that there is no reason to fear harmful effects on the chromosomes of patients from treatment with oxaminiquine.

Creatine-phosphokinase (CPK) activity in relatives of patients with X-linked muscular dystrophies: a Brazilian study

Serum CPK was measured in 135 families with Duchenne muscular dystrophy (DMD) and 19 with the Becker type (BMD). Increased CPK was found in 62% of the carriers of DMD and 62.5% of the BMD. Two certain carries of DMD and one of their daughters showed clinical signs of myopathy. Three studied DMD pregnant carriers suggest that there is a decrease in CPK levels around the 4th-5th months of gestation. In genetic counselling of suspected carriers the CPK activity of their normal daughters should always be considered. Our data suggest strongly that CPK activity decreases in carriers with increasing age.

A study of chromosomes of lymphocytes from patients treated with hycanthone

The frequency of cells presenting chromosome abnormalities was determined in patients with schistosomiasis before and after treatment with a single dose of 2.5 mg/kg body weight of hycanthone methanosulfonate. No significant effect of the drug was detected with this therapeutic dose.

New linkage data for the X-linked types of muscular dystrophy and G6PD variants, colour blindness, and Xg blood groups

Six families in which Duchenne muscular dystrophy (DMD) and G6PD or deutan colour blindness are segregating are reported. The sum of the lod-scores of these families together with three published previously indicates that the DMD locus is far from the G6PD:deutan cluster. The lod-scores of two families with Becker muscular dystrophy (BMD) informative for the G6PD locus together with those of one family previously studied by Emery, Smith, and Sanger (1969) suggest that the BMD locus could be at a measurable distance from this cluster. The maximum likelihood estimate of the recombination fraction is 0·27 and the 90% confidence limits are 0·17 and 0·40. This difference in linkage estimates for DMD and BMD suggests that the BMD and the DMD genes are located at two different loci on the X chromosome.

Five more families with DMD and two with BMD informative for Xg blood groups support the conclusion of other authors that there is no hint of linkage between the loci for Xg and for the X-linked forms of muscular dystrophy.

Searching for XYY males through electrocardiograms

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